1. Field of the Invention
The invention relates to an extruder head for the extrusion of tubular preforms having single-layer or multiple-layer walls, from plastics material which has been plasticized in extruders.
2. Description of the Prior Art
It is known to manufacture single-layer or multiple-layer plastics containers by means of blow molding from tubular preforms which are extruded by means of so-called extruder heads from plastics material which has been plasticized in extruders.
While the manufacture of preforms for containers having single-layer walls poses no problems at all, it had in the past been frequently difficult to extrude preforms having multiple-layer walls in such a way that the preforms can be used for manufacturing by blow molding plastics containers having faultless properties.
If in extruding preforms having multiple-layer walls, all these layers are formed concentrically relative to one another approximately in the same plane, the outermost layers are initially extruded with a very large diameter and must then be compressed by the influence of elastic circumferential deformations to reach the later tube diameter, while the innermost layer having the smallest diameter in the distribution system is subjected to a much smaller circumferential deformation because its diameter does not have to be changed substantially. Thus, particularly when different types of plastics materials are to be extruded in a laminar structure, it frequently happens that the layers poorly adhere to one another and the finished articles have poor properties with respect to resistance to compression and/or impact strength.
Another disadvantage is the fact that the individual extruder heads have relatively large diameters, so that these extruder heads can only with difficulty be operated next to one another as multiple extruder heads.
In order to avoid these disadvantages it is possible to place one behind the other the distribution systems in the extruder head for obtaining the individual wall layers. However, this arrangement means that the structural length of the extruder head substantially varies in dependence upon the number of wall layers to be formed, and that excessively long flow distances for the plastics material in the individual layers are required.
Thus, while the type of extruder head described first leads to extreme variations of the relative diameters of the individual layers, in the case of the second type of extruder head there exist extreme variations with respect to flow times, dwell times and shearing stresses of the individual layers.
It is, therefore, the primary object of the present invention to avoid the disadvantages of the known types of extruder heads.
It is specifically the object of the invention to provide an extruder head of the above-mentioned type in which small diameter variations between the individual layers are possible, while undesirably long flow distances and overall dimensions of the extruder head are avoided.